A function which is not defined explicitly, but rather is defined in terms of an algebraic relationship (which can not, in general, be "solved" for the function in question). For example, the eccentric anomaly of a body orbiting on an ellipse with eccentricity is defined implicitly in terms of the mean anomaly by Kepler's equation
Implicit Function
See also
Implicit Equation, Implicit Function TheoremExplore with Wolfram|Alpha
Cite this as:
Weisstein, Eric W. "Implicit Function." From MathWorld--A Wolfram Web Resource. https://mathworld.wolfram.com/ImplicitFunction.html